Peridot and Olivine: A Guide to the Raw Stone

Short answer: peridot is gem-quality olivine, (Mg,Fe)2SiO4, a mineral from Earth's mantle. Raw peridot reaches collectors in two forms: granular green nodules carried up in basalt, as at Peridot Mesa in Arizona, and rare large crystals from veins in Pakistan and Myanmar. Its color comes from iron, it shows strong doubling under a loupe, and green glass is its most common imitation.

Why is peridot the same thing as olivine?

Olivine is a group name, not a single mineral. Geology.com describes it as rock-forming minerals whose compositions typically run between Mg2SiO4 and Fe2SiO4, with magnesium and iron substituting for each other in any ratio. The magnesium end member is forsterite and the iron end member is fayalite. Collectors and dealers say "olivine" because naming the exact member takes a chemical test, and "peridot" when the material is green, transparent and clean enough to cut.

Gem peridot sits near the magnesium end. The International Gem Society explains that an iron content of just 12 to 15% gives the ideal color, while higher iron produces a muddier brown, which is why fayalite-rich olivine is not sold as peridot.

Webmineral gives forsterite a density of 3.21 to 3.33 and notes the name honors Johann Forster, a German naturalist. It also records "chrysolite" as a synonym, an old trade word for yellowish green stones that gemologists no longer use.

Where does the green come from?

Peridot is one of the few gems colored by its own main ingredients rather than a trace impurity. The Caltech Mineral Spectroscopy server states that olivine derives its color primarily from Fe2+, modified by small amounts of Fe3+, titanium and nickel, and that it is green at lower Fe2+ contents but turns brown at high iron. More iron does not mean a better green; past a point it means a worse one.

How does peridot get from the mantle to a rock shop?

Olivine is the main mineral of peridotite, the rock of Earth's upper mantle. Getting it to the surface intact is the hard part, and the two routes produce two very different kinds of specimen. The general crystal-growth background is in how crystals form.

Route one: nodules in basalt

Fast-rising basalt magma can rip pieces of mantle off the walls of its conduit and carry them up as xenoliths, foreign rock fragments inside the lava. Geology.com shows lherzolite nodules, a variety of peridotite, collected from a basalt flow at Peridot Mesa, Arizona, and notes that these xenoliths often contain olivine with color and clarity good enough for gems. GIA describes the same occurrence as irregular nodules, rounded rocks with peridot crystals inside, found in lava flows in the United States, China and Vietnam.

What that looks like on a shelf: a dark gray or black basalt chunk with a pocket or rind of sugary, bottle-green grains, each a few millimeters across. You are looking at a sample of the mantle that arrived in one eruption. The grains are crystals, but not well-formed ones; they interlock like the grains in a sugar cube. Mixed in you will often see black and dark green pyroxene, and Geology.com notes that San Carlos stones often contain visible crystals of chromite.

Arizona material is plentiful but small. Geology.com says most San Carlos stones are a few carats or less, and IGS adds that Arizona peridot over 10 carats is very rare in cut form. IGS also records a delightful detail from Arizona and New Mexico: loose peridot grains, eroded from their parent rock, turn up in the hills that ants build.

Route two: crystals in veins

The rarer route produces the specimens that make collectors pay attention. GIA notes that very rarely peridot forms as large crystals lining veins or pockets in certain types of solidified molten rock, with sources including Finland, Pakistan, Myanmar and Zabargad. Geology.com adds that the higher quality, larger crystals from Pakistan and Myanmar occur in metamorphic rocks with serpentine or dolomitic marble. GIA illustrates a 7.9 cm crystal from Sappat, Kohistan, Pakistan beside a 64.57 carat stone cut from the same deposit.

These are true crystals with faces: stubby, flattened prisms with wedge-shaped ends, sometimes gemmy end to end. A sharp, transparent Pakistani crystal over a few centimeters is a serious specimen and is priced like one.

Why does peridot weather so easily?

Olivine formed at mantle temperatures is out of its comfort zone at the surface. Geology.com notes that it is one of the first minerals to be altered by weathering, that it transforms into serpentine when metamorphosed, and that it is only abundant in sand when the source is very close. Its best known example is the green sand of Papakolea Beach in Hawaii, where olivine grains sit beside basalt and coral fragments.

For a buyer this has two practical consequences:

• Rust-brown rims on nodules are normal. The outer grains of an old basalt nodule often turn yellow-brown as iron oxidizes. A specimen with fresh green interior and a weathered crust is honest material.
• Be skeptical of "Hawaiian peridot" jewelry. Minerals.net states that Hawaii only occasionally produces peridot large enough to cut and that most "Hawaiian peridot" is not authentically from Hawaii.

How do you identify peridot with a loupe?

Peridot has two features you can see with a 10x loupe, which is more than most green stones offer.

1. Doubling. IGS notes that peridot has very strong double refraction that produces doubling and fuzziness. Look through the top of a faceted stone or a clear grain at the back edges and they appear twinned. Green glass, a frequent imitation, shows no doubling at all.
2. Lily pads. Under magnification peridot often shows flat, disk-shaped liquid and gas inclusions called lily pads, sometimes around a tiny crystal. GIA describes the same reflective disk-like inclusions and notes that tiny black spots are minute mineral crystals. The inclusions guide explains how such features form.

Other checks: weight helps, since at about 3.3 peridot feels heavier than glass of the same size. Color helps too. GIA notes the color range is narrow, from brownish green through yellowish green to pure green, with yellowish green most common.

What is sold as peridot that isn't?

IGS names green glass as the most frequently encountered peridot simulant, and lists synthetic corundum, synthetic spinel and cubic zirconia among green look-alikes. Minerals.net adds that peridot's color can resemble tourmaline and demantoid garnet. The glass imitations guide covers the bubble and mold-mark checks for the glass case.

Two natural stones cause honest confusion at a show table:

• Green tourmaline is harder and forms long striated prisms with a rounded triangular cross section, nothing like a peridot grain.
• Moldavite is natural glass. It is olive to bottle green like dark peridot, but it shows swirls and bubbles instead of doubling, and its sculpted surface looks nothing like a crystal. See the moldavite guide.

Treatment is rarely an issue for raw peridot. Minerals.net reports that peridot color is natural and not enhanced. IGS notes that some old cut stones were foil-backed, a jewelry trick rather than a treatment of the stone.

How should raw peridot be handled and stored?

At 6.5 to 7 peridot is about as hard as quartz, so it can be scratched by grit and household dust. The bigger risks for specimens are the host rock and heat. Basalt nodules are held together by the lava rind, and the green grains can loosen and pop out if the piece is dropped or scrubbed. Dust them with a soft brush, avoid soaking, and never put them in an ultrasonic cleaner. Store loose grains in a padded box rather than a jar where they grind against each other.

Peridot in crystal traditions

In crystal traditions peridot is associated with warmth, light and good fortune, a theme that goes back to its long history as a protective stone. The birthstone lore is covered in the August birthstone guide.

What to look for when you buy raw peridot

For basalt nodules, the questions are proportion and freshness: how much of the surface is green, how large the individual grains are, and whether the green is bright or brown. A stated locality matters, because Arizona nodules are a recognized classic. For crystals, look for real faces, transparency and size, and expect a large price step between grains and true crystals.

Examples currently in stock:

• Peridot on basalt from Arizona: mantle nodule specimens in matrix, sold in sizes from about 307 g to 388 g (roughly 4 to 4.5 inches long), a good way to own Peridot Mesa style material with the host rock intact.
• Green tourmaline tumbles and sphere: a useful comparison piece; set it beside peridot to see the difference between tourmaline's darker, more saturated green and peridot's yellow-green.
• Moldavite: natural green glass sold by gram weight, worth handling next to peridot to learn what "no doubling" and swirled glass look like.
• Green garnet, unpolished: another green look-alike family; garnet is singly refractive, so it never shows peridot's doubled facet edges.

Frequently asked questions

Is olivine the same as peridot?

Peridot is the gem variety of olivine. Olivine covers the whole magnesium to iron series and includes dull, brown or granular material; peridot is the green, transparent, gem-quality part of it near the magnesium end. A basalt nodule full of green grains is olivine, and the cleanest grains in it are peridot.

Why does my peridot nodule have brown spots?

Olivine weathers quickly at the surface, and iron in the outer grains oxidizes to yellow-brown. Brown rims on an old basalt nodule are a normal sign of exposure, not a fake. Judge the piece by the fresh green grains inside and by how firmly they are held in the rock.

How can I tell peridot from green glass?

Use a loupe. Peridot is strongly doubly refractive, so back facet edges or inclusions look doubled; glass shows single, sharp edges and often round bubbles. Peridot also feels heavier for its size. Disk-shaped lily pad inclusions are typical of peridot and never appear in glass.

Where does most raw peridot come from?

Basalt nodules and small grains come mainly from Arizona, with similar lava-hosted deposits in China and Vietnam. Larger crystals with faces come from veins in Pakistan and Myanmar, and these are much rarer and more expensive. Hawaii produces olivine sand, but cuttable Hawaiian peridot is uncommon.

Is raw peridot fragile?

The mineral is fairly hard, 6.5 to 7, but specimens are fragile in other ways. Grains in basalt nodules can loosen and fall out, and peridot reacts badly to heat and sudden temperature changes. Brush specimens clean, keep them out of ultrasonic cleaners, and store loose grains padded.

Sources

• Geology.com, Olivine: https://geology.com/minerals/olivine.shtml
• International Gem Society, Peridot Value, Price, and Jewelry Information: https://www.gemsociety.org/article/peridot-jewelry-and-gemstone-information/
• Webmineral, Forsterite Mineral Data: https://www.webmineral.com/data/Forsterite.shtml
• Caltech Mineral Spectroscopy Server, Olivine spectra: http://minerals.gps.caltech.edu/files/visible/olivine/index.html
• GIA, Peridot Description: https://www.gia.edu/peridot-description
• GIA, Peridot Quality Factors: https://www.gia.edu/peridot-quality-factor
• Minerals.net, Peridot gemstone: https://www.minerals.net/gemstone/peridot_gemstone.aspx